This invention relates to electrical resistor grid assemblies incorporating a plurality of forced-ventilated, welded metal plates and used in the dynamic braking or retarding function of large electric motors such as the direct-current traction motors on diesel-electric locomotives.
During dynamic braking of a locomotive, its traction motors operate in a generating mode and supply current to heavy duty resistor grids where the electrical energy is converted to heat and dissipated to the atmosphere with the aid of an associated cooling fan. The current-conducting elements of such a resistor grid are supported in a frame of insulating material having suitable dielectric and flexural strength and having a sufficiently high thermal rating to withstand the heat produced in the grid. Typical insulating material for the frame of a resistor grid assembly comprises glass fiber-filled polyester resin that was cured under pressure in a heated mold. Because there are practical limits to the thermal ratings of affordable thermosetting molding compounds, a conventional goal of resistor grid designers is to minimize the amount of heat that will transfer (by conduction, radiation, and/or convection) from the grid to the frame. Toward this end, it is common practice to space the relatively hot parts of the current-conducting elements from the insulating frame members by means of small clips, studs, or the like. The resulting free air space alongside the interior surfaces of the frame inhibit the transfer of heat from the hot elements to the insulating members, and it also allows these surfaces to be more effectively cooled by the air that is blown through the grids.
Representative prior art welded plate resistor grid assemblies are disclosed in U.S. Pat. Nos. 2,858,402, 3,550,058, and 4,654,627. In the assembly or "stack" disclosed in U.S. Pat. No. 2,858,402, each current-conducting element is a die struck strip of sheet metal having right and left-hand offset flat end portions or tangs. Adjacent strips are oppositely oriented so that the right-offset tang of one converges with the left-offset tang of the other. These mating tangs are united by soldering or welding and are inserted in a slot of an insulating support member.
In the assembly disclosed in U.S. Pat. No. 4,654,627, each current-conducting element is a metal plate having oppositely offset planar ends each of which has two narrow, integral, concave projections extending outwardly in a longitudinal direction therefrom. When a first one of the offset ends of each plate is welded to the other offset end of an adjacent, oppositely oriented plate, the two projections of the first end respectively mate with the two projections of the other end of the adjacent plate to form a pair of generally cylindrical mounting legs which are respectively inserted in a corresponding pair of blind holes in an associated insulating support member. The aforesaid legs are long enough to "bottom out" in the holes that respectively receive them, thereby positioning the offset ends of adjacent welded plates away from the support member.
So long as its manufacturing process is carefully controlled, the resistor grid assembly disclosed in the above-mentioned U.S. Pat. No. 4,654,627 has proven satisfactory. Good quality control practices are required to obtain the requisite alignment and mating of the narrow, concave projections, to keep each pair of mating projections from spreading apart during the welding process, and to avoid external burrs or sharp edges that might bite into the bottom or side of the cooperating hole and therefore interfere with uniform bending of adjacent plates during thermal excursions. Such quality measures (including relatively frequent maintenance and resharpening of the tooling used to form the mounting legs) have an adverse effect on the manufacturing cost of the plates used in the assembly. Furthermore, inserting all of the separate mounting legs into their respective cooperating holes during the process of installing the welded resistor grid in the frame of this prior art assembly has been undesirably difficult and time consuming. Another shortcoming of this assembly is that the diameter of each hole in the insulating support members needs to match the outside diameter of each cylindrical mounting leg which in turn is a function of the thickness or gauge of the metal plates, whereby different frames are required for resistor grids of different current ratings.
Adjacent plates of the assembly disclosed in U.S. Pat. No. 4,654,627 are joined together by seam welds. Heretofore such seam welds have been obtained by a rapid-fire, multiple-spot resistance welding process. The adjoining offset ends of the pair of plates to be welded are temporarily clamped together, and this workpiece is advanced transversely in a series of short, incremental steps through the resistance welder. In each step, the workpiece pauses while it is clamped between the welder electrodes to which a pulse of electrical current is then applied. The resulting seam weld actually comprises a chain of overlapping spot welds. This process is inherently slow and is sensitive to variations in both the surface and the internal resistance of the workpiece. Consequently, careful preparation and control are required to ensure uniformly good spot welds in the chain of such welds forming the seam weld.